Modern automotive ignition systems typically rely on an ignition coil to generate spark energy, where the spark energy is used by a spark plug to generate an ignition spark in an engine cylinder, thereby initiating fuel combustion.
Generally speaking the greater the spark energy produced the better fuel will combust, thereby increasing the reliability of cold starts, plus more stable combustion and more stable idle.
To allow spark energy to be generated requires the energization of an ignition coil, where the energization of an inductive ignition coil is typically controlled by the provision of a control signal to a coil current switching element that is arranged to allow current to flow through a primary winding of the ignition coil during the energization phase. The spark energy is generated when the control signal to the coil current switching element causes the flow of current through the primary winding to stop.
The spark energy generated by an inductive ignition coil is determined by the magnetic flux of the ignition coil, which in turn is determined by the inductance of the ignition coil and the current flow through the primary winding of the ignition coil at the point current flow is stopped.
However, with the constant drive to reduce the cost and weight of automobiles the ignition coil, and in particular the iron core of the ignition coil, has inevitably been reduced in size and weight and as a consequence the maximum amount of magnetic flux that inductive ignition coils can now produce has consequently also been reduced.
Consequently, to maximise the spark energy produced by an ignition coil it is desirable to increase the current flow through the ignition coil to the point where the iron core of the ignition coil is saturated, thereby maximising magnetic flux. The time period over which current flow through the ignition coil occurs is commonly called dwell time.
If the current flow through the primary winding of the ignition coil is insufficient to saturate the iron core the magnetic flux, and consequently spark energy, is reduced from its maximum value by the square of the difference between actual current flow and current flow required for saturation.
An increase in the flow of current beyond saturation will have no additional effect on spark energy as magnetic flux will not increase beyond its saturation point. However, an increase in current flow beyond saturation can result in excessive current flow through the coil current switching element, which can result in damage to the coil current switching element and/or to the ignition coil itself.
Various solutions for determining optimum current flow have been proposed. For example, direct measurement of the current rise through the coil until it reaches a predetermined threshold with a resulting dwell time is a long standing solution (for example U.S. Pat. No. 4,198,936). However, this has at least four drawbacks: firstly the current threshold is fixed by component values which have to be adjusted for each subsequent ignition system; secondly the current threshold must always be set conservatively to accommodate variation in the magnetic flux capability of the ignition coil; thirdly it is an expensive solution because it relies on an accurate means to measure said current; and fourthly there is no mechanism to compensate for speed variation during the dwell cycle. Ignition systems aim to deliver the spark energy at a particular crank angle. Since ignition controllers typically work in time, the controller will convert this future angle into a future time. Any engine speed change subsequent to commencement of dwell will result in an angle error for the ignition event unless the controller changes the dwell time to compensate for the engine speed change. If the dwell time is changed then the coil current will not be optimum.
Improvements have been made, for example U.S. Pat. No. 6,750,565 describes a time extrapolation method that is based on a measured current threshold significantly lower than that for maximum flux. This eliminates the first of the above four problems but still suffers from the second, third and fourth problem.
U.S. Pat. No. 6,595,192B1 eliminates the current measuring circuitry through modelling of the appropriate dwell time as a function of battery voltage: this eliminates the first and third problems but still suffers from the second and fourth problem.
U.S. Pat. No. 6,100,728 uses current limiting hardware to hold the current to a known value. This eliminates the fourth problem as the target current can be reached and held until the ignition event is due to occur. However, this still suffers from the first, second and third problems and in addition incurs significant heat dissipation in the switching element for the period when the current is held.
Additionally, a further requirement of a modern ignition system is to provide diagnostic feedback in order to comply with emissions legislation. The prior art systems provide limited diagnostics.
Accordingly, it is desirable to provide a solution that allows optimisation of current flow through an ignition coil during energization of the ignition coil irrespective of operating conditions.